1. Field of the Disclosure
The present application relates to an organic light emitting display device. More particularly, the present application relates to an organic light emitting display device adapted to enhance high temperature reliability and expand life time of elements by adjusting hole and electron mobilities of a hole transport layer and an electron transport layer which are formed in an organic light emitting diode.
2. Description of the Related Art
An organic light emitting diode used in an organic light emitting display device is a self-luminous element which includes an emission layer formed between two electrodes. The organic light emitting diode generates excitons by injecting electrons and holes into the emission layer through an electron injection electrode (i.e., a cathode) and a hole injection electrode (i.e., an anode) and recombining the electrodes and the holes within the emission layer. Also, the organic light emitting diode emits light when the excitons are transitioned from an excited state into a ground state.
The organic light emitting display device using the organic light emitting diode is classified into a top-emission mode, a bottom-emission mode and a dual-emission mode according to the light emission directions. Also, the organic light emitting display device can be divided into a passive matrix type and an active matrix type.
In order to display an image, the organic light emitting display device can apply scan signals, data signals and supply voltages to a plurality of sub-pixels, which is arranged in a matrix shape, and enable selected sub-pixels to emit light.
Also, in order to enhance luminous efficiency and color coordination of a display panel, organic light emitting display devices with a micro-cavity structure which allows red, green and blue sub-pixels to be formed differently from one another in thickness have been developed.
FIG. 1 is a cross-sectional view showing the structure of an organic light emitting diode which is formed in a sub-pixel region of an organic light emitting display device according to the related art.
The organic light emitting diode shown in FIG. 1 corresponds to an organic electronic element which converts electrical energy into light energy. Such an organic light emitting diode includes an organic emission layer EML interposed between an anode electrode E1 and a cathode electrode E2 and configured to emit light. The anode electrode E1 is used to inject holes, and the cathode electrode E2 is used to inject electrons.
The electrons and the holes injected from the two electrodes E1 and E2 are drifted into the organic emission layer EML and form excitons. The electrical energy of the excitons is converted into visible light so that visible light is emitted. In order to easily and smoothly inject the electrons and the holes from the two electrodes E1 and E2 into the organic emission layer EML, not only a hole injection layer HIL and a hole transport layer HTL are formed between the organic emission layer EML and the anode electrode E1, but also an electron transport layer ETL and an electron injection layer (EIL; not shown) are formed between the organic emission layer EML and the cathode electrode E2.
In general, an electrical charge distribution zone where holes and electrons exist is formed within the organic emission layer EML of the organic light emitting diode. Also, the electrical charge density has the largest value in the central region (or axis, hereinafter ‘peak electrical-charge density region (or axis)’) of the electrical charge distribution zone and is gradually lowered as it goes to edges of the electrical charge distribution zone.
With the exception of the electrical charge distribution, a hole-electron recombination zone, in which the holes and the electrodes are recombined with each other, is formed within the organic emission layer EML. The organic light emitting diode is generally manufactured in such a manner that the central region (or axis) of the hole-electron recombination zone overlaps with the peak electrical-charge density region (or axis). The central region (or axis) of the hole-electron recombination zone corresponds to the highest region (or an axis; hereinafter ‘peak recombination density region (or axis)’) of the density of holes and electrons which will be recombined with each other.
The peak electrical-charge density region (or axis) can be fixed to the central region (or axis) of the organic emission layer EML according to the formation material of the organic emission layer EML. The hole-electron recombination zone can be shifted by dispersion degrees of the holes and electrons which are drifted into the organic emission layer EML for the recombination with each other.
In order to overlap the peak recombination density region (or axis) with the peak electrical-charge density region (or axis), the organic light emitting diode generally allows the organic emission layer EML and the hole and electron transport layers HTL and ETL to be adjusted in thickness.
FIG. 2 is a graph illustrating a life time characteristic of an organic light emitting diode according to the related art. FIG. 3 is a graph illustrating a brightness characteristic of an organic light emitting diode according to the related art after a high temperature reliability inspection.
As seen from FIGS. 2 and 3, it is clear that the life time of a red organic light emitting diode is reduced with the lapse of time, and brightness of the red organic light emitting diode after a high temperature reliability inspection gradually deteriorates with the lapse of time.
In the drawings, a characteristic curve ‘-●-:Red’ represents variations of the current efficiency (cd/A) of the organic light emitting diode, which is driven at the room temperature, with the lapse of time. Another characteristic curve ‘---:R-after 240 Hr’ represents variations of the current efficiency (cd/A) of the organic light emitting diode which is driven at a high temperature, with the lapse of time. Based on the comparison between the two current efficiency characteristic curves, it is clear that the current efficiency of the organic light emitting diode deteriorates after a high temperature reliability inspection.
The current efficiency of the organic light emitting diode affects brightness of the organic light emitting diode. Due to this, the brightness of the organic light emitting diode also deteriorates after the high temperature reliability inspection.
The inventors of the present application have found out that this deterioration results from the fact that the organic emission layer of the organic light emitting diode is formed in an overlapping shape of the peak recombination density region (or axis) and the peak electrical-charge density region (or axis), and the hole-electron recombination zone tends to expand toward low electrical-charge regions when the organic light emitting diode is driven.